“Baby, It’s Cold Outside”

Brrr! This been a bitterly cold winter in the Midwest. The deep freeze has stuck around for most of the last two months too. Yet strange as it seems, this winter’s weather is consistent with climate change.

So says Jennifer Francis, a climate researcher at Rutgers University. Yes, this winter has indeed been very cold here in the United States. But that’s not all. “The bigger story, if you sit back and look at the whole northern hemisphere, is warmth,” she says. Sweden, for example, has been unusually warm this winter.

Arctic warming might explain why, suggests Francis. Earth’s overall average temperature has risen only slightly over the last 100 years. However, the most dramatic warming has taken place in the Arctic. The Northwest Passage that explorers sought over 200 years ago has already opened up, at least in the summer months. Meanwhile, scientists are seeing dramatic losses in sea ice and impacts on ecosystems throughout the Arctic.

When the Arctic is less cold, there’s less difference between it and the mid-latitudes that include much of the United States. The difference between the two regions is one of the things that drives the westerly wind known as the jet stream. When there’s a bigger difference between warm and cold regions, the jet stream is faster and flows pretty directly from east to west.

Reduce the temperature difference between the two areas, and the jet stream should flow slower. And slower flow could let the jet stream meander northward and southward in a wavy pattern.

“I like to think of it as a river,” says Francis. A river flowing swiftly downhill tends to follow a fairly straight course. Once it comes to a more level plain, it’s more likely to meander along a winding course around whatever obstacles are in the way.

Picture that happening throughout the northern hemisphere with a huge river of air. Air flowing up from the south would bring warmer weather. Air flowing down from the northernmost parts of the wavy pattern would bring colder weather.

The phenomenon is relatively new, says Francis. And with only about 15 years of data, she’s not yet ready to say this winter is definitely due to Arctic warming. But, she says, what we’re seeing is consistent with her hypothesis.

Francis discussed her research at the American Association for the Advancement of Science’s annual meeting in Chicago on Saturday. She and colleague Steve Vavrus of the University of Wisconsin initially published their hypothesis in 2012 in Geophysical Research Letters. Francis also presented information about the research at the 2011 American Geophysical Union meeting in San Francisco. New data collected since then continue to support her hypothesis.

Not everyone accepts Francis’s view, though. One challenge came last year from Colorado State University researcher Elizabeth Barnes. She argued that the results found by Francis and her colleagues could have had been a weird result, an “artifact” of their research. Francis disagreed, although she admits that there’s not yet enough data to get to the 95 percent confidence level that most scientific researchers look for. Nonetheless, she and other scientists have been collecting more data. And, Francis says, it’s “getting close.”

Perhaps just as importantly, no one has offered any better explanation for some of the phenomena we’ve seen this winter. Nor has anyone come forward with any evidence to contradict her hypothesis.

Francis and other scientists will continue to collect and analyze data to determine how much of an effect Arctic warming is really having on lower latitudes. Meanwhile, be sure to button up your overcoat—at least if you’re in the Midwest. As the old song goes, “Baby, it’s cold outside!”